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Camphor Mediated Combustion and Sublimation: A Unique Approach in Articulation of Enhanced Defects in Pristine MWCNTs Suneel Kumar Srivastava, and Kunal Manna J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b04507 • Publication Date (Web): 28 Jul 2017 Downloaded from http://pubs.acs.org on July 28, 2017
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Suneel Kumar Srivastava†,* and Kunal Manna† †
Department of Chemistry, Indian Institute of Technology, Kharagpur-721302, India
ABSTRACT: Additional defects inclusion in the form of rehybridization, twisted kinks and fragmentation, has been achieved in MWCNTs fabricated through a unique camphor mediated combustion and sublimation approach. The significant defects induced in MWCNTs through combustion of camphor is unambiguously evidenced by its superior fluorescence quenching efficiency. 1. INTRODUCTION Physical properties of materials are largely influenced by the nature and extent of the defects present in their atomic arrangements.1 Therefore, tuning the defect is critical for specific application of a material worth exploring and researching.2 Multiwalled carbon nanotubes (MWCNTs) are one such example and most investigated 1D material for their applications in different fields.1 During growth of pristine MWCNT, structural disorders referred as diverse intrinsic reticular imperfections manifest, such as Stone-Wales defects, atomic defects in the hexagonal C–C network, vacancies and divacancies (Frenkel type pairs), heptagon–hexagon pairs, pentagon-octagon-pentagon pairs, incomplete bonding defects (folding, dislocations, dangling bonds), edge-like defects, tube kinks, interstitial defects between external and internal tubes at the graphitic lattice etc.3 These invoked structural deformations facilitate high chemical and electrochemical activity which is ascribed to an enhanced adsorption ability of the defective MWCNTs due to presence of high density of active nucleating centers.4 In addition, post synthesis treatment of MWCNTs also introduce various types of defects, such as strain, adatoms, doping with other elements than carbon, re-hybridization (electronic state between sp2 and sp3), fragmentation etc.5 The presence of these intrinsic and induced defects in MWCNT influences their applications seriously in many electronic devices.6 The presence of these intrinsic and induced defects retards mechanical strength 6 and electronic properties of MWCNT6 influencing their applications in many devices. Additionally, buckling capacities,7 yield strength,8 elastic modulus,9 conductivity10 and transport properties11 of CNTs are also adversely affected. Despite these drawbacks, defects in MWCNT has led numerous advantages in field of sensoristic,4,12,13 catalyst in fuel cells,6,14 electrochemistry,8 electrodes for fast electron transfer kinetics,15 biochemical sensors,16 high radiation absorber, 17,18 emission devices,8,19,20 supercapacitors,8,21 lithium ion batteries,1,22 hydrogen storage etc.8,23 Furthermore, defects upsurge the interfacial bonding strength between MWCNT and their surrounding polymer matrix 24 facilitating the load transfer among different layers in MWCNTs.25 Moreover, conversion of nanotubes from one diameter to another, and on Y junction in molecular electronics is also assisted by defects.26 Therefore, several physical and chemical methods have also been reported in introducing structural defects in pristine MWCNT. In physical methods, defects are generated in MWCNT by ball milling,6 ion,27γ-ray,28and 60Co irradiation,29 plasma etc.30 Alternatively, chemical methods of defect generation in MWCNT has also been reported, such as surface functionalization,31 acid treatment,32 hydroxylation,6 oxidation etc. In addition, pyrolysis of hydrocarbons at elevated temperature in presence of metal catalyst also introduced defects in MWCNTs (Table S1).18 However, many vital issues, such as disposal of waste containing MWCNT33 and use of highly
toxic aromatic organic compounds involved in functionalization of MWCNTs still remain to be addressed.34 Therefore, simple facile, mild and straight forward method is desirable in generating substantial defects in MWCNT while avoiding excessive interruption in its basic structure and property. In view of this, we report a unique approach of achieving additional defects in intrinsically defective pristine MWCNT through simple camphor mediated combustion and sublimation of MWCNT:Camphor (1:20 wt/wt) mixture based on fundamental concepts of heat transfer. The choice of camphor in present work is mainly guided by nontoxicity,35 flammable under ambient conditions (25C, 1 bar),35 low flash point (53C),36 higher standard heat of combustion37 and its lower flame temperature compared to thermal stability of MWCNT in air (~500 0C).38,39 Therefore, it is anticipated that heat released (ΔcomH0solid = -5903 kJ mol-1) in the combustion of camphor could generate defects in MWCNT. Alternatively, possibility of defect introduction through sublimation of camphor (ΔsubH0 = 51.8 kJ mol-1) below its triple point (1790C) also cannot be ruled out. Such novel approach of combustion and sublimation of camphor is also well supported by thermodynamics (SI-1).40 Pristine and synthesized defective MWCNT through camphor mediated combustion (MWCNT-CM) and sublimation (MWCNT-SB) were characterized by Field Emission Scanning Electron Microscopy (FESEM), High Resolution Transmission Electron Microscopy (HRTEM), Atomic Force Microscopy (AFM), X-Ray Diffraction (XRD), X-Ray Photoelectron Spectroscopy (XPS), Raman and BET surface area measurements to establish formation of segmented and defective MWCNTs through combustion and sublimation of camphor (SI-2). The investigation of photophysical properties of pristine MWCNT and defective MWCNT as quenchers through fluorescence quenching of fluorescein also validated following order of defects density: MWCNT: MWCNT-CM (camphor combustion) >> MWCNTSB (camphor sublimation) >MWCNT (Pristine). 2. EXPERIMENTAL SECTION 2.1 Materials. MWCNTs, camphor and Fluorescein were purchased from Sigma Aldrich and Exciton respectively. Ethanol, methanol and acetone were procured from SRL Pvt. Mumbai. All the regents were used without any further purification. 2.2 Fabrication of Camphor mediated defective MWCNTs. A mixture of 20:1 (wt./wt.) camphor and MWCNT was thoroughly mixed with the help of spatula in a porcelain pot and ignited with a burning matchstick. The combustion of the camphor continued till the burning stopped automatically. In another experiment, thoroughly mixed camphor and MWCNT mixture (20:1 wt./wt.) was placed inside a porcelain disc and covered it with an inverted funnel. The camphor in the mixture was subsequently allowed to undergo complete sublimation at 145 OC. The products left after combustion and sublimation were referred as MWCNT-CM and MWCNT-SB respectively.
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3. RESULTS AND DISCUSSION Morphology of MWCNT-CM and MWCNT-SB were investigated by FESEM, TEM and HRTEM to provide insight on the structural defect induced in pristine MWCNT and corresponding images are displayed in Figure 1 (a-b), (c-d) and (e-f) respectively. The zoomed images in insets indicated generation of relatively more intense defects in MWCNT-CM compared to either MWCNT-SB or pristine MWCNT. This could be ascribed to the formation of disordered nanotubes due to random breaking of the nanotube walls and inhomogeneity of wall thickness in MWCNT-CM. TEM image of MWCNT-CM also clearly demonstrated deposition of disordered carbon layers on the outer side walls of MWCNT. In addition, it revealed manifestation of re-hybridization defects resulting from the destruction and conversion of sp2 bonded network into sp3 of C–C bonds. TEM study also established fragmentation of pristine MWCNTs to shorter nanotubes with more open ends tubes. 6 High resolution TEM images also prominently demonstrated twisting of MWCNT tube kinks and fragmentation of nanotubes in MWCNT-CM and MWCNT-SB respectively. Figure S1-(d), (e) and (f) provide information on lattice fringes in pristine MWCNT, MWCNT-CM and in MWCNT-SB respectively. It is noted that straight rectilinear fringes in pristine MWCNT account for its less defective structure.41 The interplanar distance in fringes corresponds to ~0.3361 nm (002 plane) are in agreement with that deduced from XRD.42 Further, no notable distortions are observed in the inner walls of the pristine MWCNTs and majority of planes stay aligned.42
(e)
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In contrast, appearance of wavy and discontinuous lattice fringes in MWCNT-CM corresponds to the signature of distorted graphitic layers.42 Further, an increased interplanar distance (as discussed in XRD) in MWCNT-CM (0.3386 nm) and MWCNT-SB (0.3365 nm) could be attributed to interlayer interactions, change in helix angle and defect mediated structural stabilization.43 Thus level of defects generated in pristine MWCNT in camphor mediated samples follows the order: MWCNT-CM > MWCNT-SB > MWCNT. In all probability, maximum defects in MWCNT-CM sample arises from the transfer of heat released in combustion of camphor to MWCNTs. It may be interesting to add that point defects (vacancies, dislocations) can easily be annealed out by heat treatment.43 In contrast, additional defects are generated in MWCNT-CM and MWCNT-SB compared to the point defects existing as intrinsic defects in pristine MWCNT. Therefore, it is anticipated that camphor mediated defects in MWCNT-CM and MWCNT-SB could play a vital role in anchoring multiple functionalities acting as nucleating sites in directing them for their specific applications. The role of defects in enhancing dispersion of MWCNT in aqueous medium has also been investigated and corresponding findings are displayed through respective digital images in Figure 2. It is observed that MWCNT-CM and MWCNT-SB showed significantly enhanced dispersion compared to pristine MWCNT due to enhanced defect density.41 These findings were further reaffirmed through AFM analysis (Figure S2). It is noted that surface roughness follows the order: MWCNT –CM (2.20 nm) < MWCNT-SB (5.44 nm) < MWCNT (7.38 nm). This could be attributed to a lower degree of entanglements of MWCNT as a result of reduced intra-bundle interaction.3 Further, highest de-aggregation of MWCNT-CM as separated tubes in ethanol could be endorsed by the interplay of superior dispersibility on account of higher induced defects in the form of small bundles.3
(f) (C) (d)
(c)
(B)
(b)
(a) (A)
Figure 1. (A) FESEM image of (a) MWCNT-CM and (b) MWCNTSB. (B) TEM image of (c) MWCNT-CM and (d) MWCNT-SB. (C) HRTEM image of (e) MWCNT-CM and (f) MWCNT-SB.
Figure 2. Digital images showing aqueous dispersion of 1 mg/mL of (a) MWCNT-CM, (b) MWCNT-SB and (c) pristine MWCNT at room temperature (Photographs recorded after 1 day).
X-ray diffractograms (XRD) of pristine MWCNT, MWCNTCM and MWCNT-SB is displayed in Figure 3 (a). A 002 diffraction peak appeared in all samples corresponding to d 002 values of 0.3361, 0.3365 and 0.3386 nm due to graphite structure in pristine MWCNT, MWCNT-CM and MWCNT-SB respectively. Full width at half maximum (FWHM) calculation
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(a)
(b)
Figure 3. (a) XRD and (b) XPS of MWCNT, MWCNT-CM and MWCNT-SB.
showed the following order: MWCNT-CM (11.51) >> MWCNT-SB (8.31) ≈ MWCNT (8.30). XRD also showed that intensity of (002) peak is significantly reduced in MWCNT-CM and MWCNT-SB compared to pristine MWCNT. This could be attributed to the presence of finite number of layers and of the curvature of each tube. 44 The microstrain (ɛ) in MWCNT and camphor mediated MWCNTs were calculated.2 It followed the order: MWCNT-CM (0.231) > MWCNT-SB (0.147) > MWCNT (0.145). This suggests that significant increase in microstrain introduced substantial defects in MWCNT-CM.3 It is also noted that degree of graphitization is lowest in MWCNT-CM (~63%) corresponding to either MWCNT-SB (~86%) or pristine MWCNT (~92%). This further strengthened our earlier contention based on FESEM and TEM on maximum deformation observed in MWCNTCM.45 XPS analysis has been used to study the bonding environment of the carbon atoms on the surface of pristine and defective MWCNTs. The corresponding images in Figure 3 (b) show the C1s envelope of pristine MWCNT, MWCNT-CM and MWCNT-SB. The shift in binding energy of C1s signal in camphor mediated combustion and sublimation follow the order: MWCNT-CM (284.15 eV) > MWCNT-SB (283.88 eV) ≈ MWCNT (283.87 eV). Such shifting of binding energy in MWCNT-CM has also been observed in covalent functionalization, such as hydroxylation,6 acid treatment32 and plasma treatment 46 due to defect generation in MWCNT. Therefore, it could be concluded that camphor mediated combustion process without involving any hazardous chemicals introduced considerable defects in pristine MWCNTs. Figure S3 depicts typical de-convoluted high-resolution C1s spectra of the pristine MWCNT, MWCNT-CM and MWCNT-SB. The de-convoluted peaks in pristine MWCNTs correspond to graphitic sp2-hybridized carbon (283.87 eV), C = C (284.67 eV), sp3-hybridized defects, including hydrocarbons (285.25 eV) and π−π* “shakeup” transitions (290.13 eV).33 It is noted that the width and intensity of sp2 peaks in MWCNT-CM (284.26 eV) and MWCNT-SB (283.98 eV) decrease, while intensity of sp3 peaks in MWCNT-CM (285.22 eV) and MWCNT-SB (285.38 eV) increase compared to pristine MWCNT. Further, it is also noted that intensity of peaks corresponding to sp 3 hybridized carbon is much higher in MWCNT-CM than MWCNT-SB and pristine MWCNT. All these observations reveal the inclusion of relatively more intense additional defects (rehybridization) in MWCNTs by means of combustion rather than sublimation process of camphor in MWCNT/camphor mixture. The peak
(FWHM-3.8) at 290.13 eV in pristine MWCNTs corresponding to π-π* shake-up satellite is sensitive towards its aromatic nature.33 The broadening of this peak (FWHM value ~4) in MWCNT-CM is a clear indication of sp3 defects present in MWCNT-CM in the form of amorphous carbon.33 Raman spectrometry analysis was carried out to quantitatively analyze and identify the role of any additional defects, if any, in pristine MWCNT compared to its camphor treated product through sublimation and combustion processes. Raman spectra of pristine MWCNT, MWCNT-CM and MWCNT-SB are displayed in Figure 3. The appearance of characteristic D band (MWCNT:1346 cm-1, MWCNT-CM:1345 cm-1, MWCNTSB:1343 cm-1) and the G bands (MWCNT:1583 cm-1, MWCNT-CM:1578 cm-1, MWCNT-SB:1576 cm-1) are clearly inevitable from the corresponding spectra. It is evident from the spectra that intensity of the D band in the camphor treated MWCNTs is relatively enhanced compared to pristine MWCNT which clearly indicated the presence of relatively more defects in MWCNT-CM and MWCNT-SB. This has also been further examined by calculating the intensity ratio (ID/IG) of each sample after extracting the areas under the D and the G peaks from Raman spectra.6,47-51 It is noted that ID/IG value of pristine MWCNT (1.22) is increased substantially in MWCNTSB (1.35) and MWCNT-CM (1.45). This clearly indicated the presence of enhanced defect concentration in camphor mediated combustion and sublimation respectively, with respect to pristine MWCNT.27 Raman spectra also showed downshift of G-peak in MWCNT-CM (∼5 cm-1) and MWCNT-SB (~7 cm-1) in comparison to pristine MWCNT in all probability following the degradation of the intra-bundle interactions due to defect induced surface modifications of the nanotubes.
D
G
Figure 4. Raman Spectra of MWCNT, MWCNT-CM and MWCNT-SB.
BET surface areas of MWCNT, MWCNT-CM correspond to 257 and 245 m2 g-1 respectively. The reduced BET surface area in defective MWCNTs can also be correlated with the lower aspect ratio of defect induced MWCNTs due to its fragmentation upon burning with camphor.52 The observed decrease in surface area and microporisity (pore diameter 246-300 Å and 248-321 Å for MWCNT-CM and pristine MWCNTs) in the defect induced MWCNTs could be accounted to the higher degree of de-aggregation as earlier observed in AFM analysis. Figure 5 (a), (b) and (c) show steady-state PL spectra about the quenching effect of different concentrations of MWCNTs, MWCNT-CM and MWCNT-SB. It is noted that the intensity of
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emission band (513 nm) corresponding to Fluorescein decrease with increasing concentration of MWCNTs. The quenching efficiency of these different quenchers was also calculated through Stern-Volmer plots as depicted in Figure 5 (d). This shows MWCNT-CM exhibits highest quenching efficiency compared to MWCNT-SB and pristine MWCNTs due to presence of higher defect density.53-55 The non-linear nature in Stern–Volmer plot indicates the possible involvement of combined dynamic and static quenching. 53 According to available literature, combined effect of static quenching and dynamic quenching of fluorescein could be ascertained through lifetime measurements. In view of this, following non-linear equation in concentration for a fluorophore quenched by collisions and by complex formation both could be used: 𝐹0 𝐹
= (1 + 𝐾𝐷 [𝑄])(1 + 𝐾𝑆 [𝑄]) = 1 + 𝐾𝑎𝑝𝑝 [𝑄]
(1)
,where KS, KD represents static and dynamic Stern–Volmer constants, F0/F represents the fluorescence remaining given by the product of fraction not complexed and fraction not quenched by collisional encounters.53 The deviation from standard SternVolmer equation in our case could be well explained on the basis of limited accessibility of bundled MWCNTs as quencher. The observation of such non-linearity in the Stern-Volmer plots could be ascribed to the outcome of several other factors, such as static quenching, dynamic quenching, possibility of FRET, higher probability of quenching represented by sphere of action model and rearrangement of fluorophore molecules.56,57
Figure 5. Steady-state PL spectra showing the quenching effect on fluorescein with different concentrations of (a) MWCNTs, (b) MWCNT-CM, and (c) MWCNT-SB. (d) Comparison of quenching efficiency for MWCNTs, MWCNT-CM, and MWCNT-SB through Stern-Volmer plots with a compressed-exponential fit.
Interestingly, corresponding Stern-Volmer plots of MWCNT followed compressed exponential growth behavior in accordance with the equation given below: 𝐹0 𝐹
= exp(𝑏[𝑄])
𝛼
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Though, Stern Volmer plots depicted analogous quenching behavior of MWCNTs on the Fluorescein, relative quenching efficiency follows the order: MWCNT-CM (80.6%) > MWCNTSB (53.5%) > MWCNT (33.3%) (Table S2). The extent of such large quenching could not be accounted in terms of usual SternVolmer equation. However, this phenomena has been interpreted on the basis of sphere of action model assuming the probability of quenching to be unity and effect of dynamic quenching to be negligible (SI-3). This concept of sphere of action model could be extended to the long cylindrical morphology of MWCNTs acting as quenchers in explaining the compressed exponential growth behavior in quenching of fluorescence. It is anticipated that relatively larger volume of MWCNTs compared to spherical quencher could contribute significantly towards the quenching of fluorophore. Our findings on compressed exponential fit suggested that quenching efficiency of MWCNT-CM (α=0.4500) and MWCNT-SB (α=0.62242) follow faster exponential growth behavior compared to pristine MWCNT (α=0.79754). Such observations could be accounted due to the presence of additional defects in MWCNT-CM and MWCNT-SB in the form of strain, curvature, fragmentation, helicity of the lower diameter MWCNTs induced on pristine MWCNTs. As a consequence, these induced defects fine-tune the reactivity and oxidative stability of the nanotubes that accounts for the observed high efficiency quenching of fluorescein by MWCNT-CM and MWCNT-SB.53 Further, restricted possibility of local rearrangement of fluorescein in the proximity of the defect induced MWCNTs or micro-collapse of quenchers for observed compressed exponential growth also cannot be ruled out.53 Additionally, the larger area of action from the extended cylindrical geometry54 and high strain55 are likely to provide large number of highly reactive sites for the adsorption of fluorophore on the defect induced MWCNTs for its quenching. It is concluded that induced defect generated in camphor treated MWCNT exhibited relatively higher quenching efficiency compare to pristine MWCNT. A monotonic decrease in the PL life time of fluorescein has been observed on addition of MWCNT-CM, MWCNT-SB and pristine MWCNT (SI-4). This is most likely due to the preferential diffusion of the fluorescein on the defect induced MWCNTs during the lifetime of the excited state.55 Further, minor alteration in τ0/τ (Figure S4) infers insignificant contribution of dynamic quenching in the fluorescence quenching. 4. CONCLUSIONS In summary, XRD, Raman and XPS showed that one step, facile process adopted for the generation of additional defects in MWCNTs follows the order: MWCNT-CM > MWCNT-SB > MWCNT. PL study also demonstrated that extent of defects in MWCNTs tune the quenching efficiency. Such enhanced structural defects generated in pristine MWCNT could find better applications in many fields.
Characterization Techniques, Sphere of Action Model, Time Resolved Fluorescence Spectroscopy, FESEM, TEM, HRTEM images of pristine MWCNT, Lattice fringes and AFM imagesof pristine MWCNT, MWCNTCM and MWCNT-SB, deconvoluted XPS spectra, UV-absorption spectra, spectral overlap of FRET, Literature Review on defects of MWCNTs and
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Comparative Table of Quenching Efficiency of the defect induced MWCNTs with Literatures.
*(S.K.S.) E-mail:
[email protected]. Department of Chemistry, Indian Institute of Technogy, Kharagpur, India, 721302.
Equal contributions have been made by the authors in performing experiments, writing and review of this manuscript.
The authors declare no competing financial interests.
Authors would like to thank Prof. Sanjeev Kumar Srivastava and Prof. Nilmoni Sarkar of Department of Physics and Chemistry for XPS and fluorescence measurements respectively. Acknowledgements are also due to Mr. Indrajit Srivastava for his valuable suggestions on this manuscript.
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